The present invention relates to nuclear-based contraband detection systems, and more particularly to an apparatus and method for detecting contraband concealed within a container, such as a suitcase, parcel or other object. As used herein, the term "contraband" includes, but is not limited to, explosives, drugs, and alcohol.
There is a pressing need in the airline industry for a system and/or method that expeditiously scans luggage and parcels to detect explosive material. It is obvious that in the use of such a system or method, the probability of explosive detection must be reassuringly high. Furthermore, because of the large number, close to two million pieces of luggage, that are checked and/or carried daily onto aircraft across the country, the occurrence of false alarms should be sufficiently rare in order to avoid nuisance to the public.
There is a similar urgent need in the customs and law enforcement fields for a like system and method that reliably detects other contraband material, e.g., drug hidden within parcels or baggage in transit across international borders. Such a system and/or method must also demonstrate a high probability of detection and a low probability of false alarms.
To meet these challenges, highly sensitive, specific, fast, and non-intrusive detection techniques must be applied. The appropriate nuclear based techniques satisfy these requirements. They provide means for rapid and non-intrusive interrogation of objects and, when properly designed, assure negligible biological hazard.
Diagnostic nuclear techniques in general involve use of two highly penetrating radiations (neutrons and gamma rays) which enable one to detect concealed explosives or other contraband materials. The radiations act as follows: An appropriately fashioned primary radiation excites atomic nuclei within a designated volume of an object. The excited atomic nuclei subsequently relax, emitting electromagnetic or particle radiation that is characteristic of the nuclear species. The analysis of the emitted spectrum thus facilitates the detection of a particular substance within the object, e.g., explosives or illegal drugs. That is, if the emitted spectrum includes radiation of a given energy, then the presence of a particular element within the object can be inferred. Thus, a particular spectrum showing characteristic radiation lines of particular intensities serves as a "signature" that identifies the presence of a particular chemical element within the object being examined. Identifying the chemical elements and/or chemical compounds within an object thus involves identifying the corresponding signatures that are present in the radiations emitted from the material as described, e.g., in Gozani, Active Nondestructive Assay of Nuclear Materials, United States Nuclear Regulatory Commission, NUREG-CR-0602, SAI-FM-2585 (1981).
It is common practice to use neutrons as the primary radiation and to measure the ensuing gamma-ray spectra for the non-intrusive diagnostic purposes. U.S. Pat. No. 3,832,545 and patent application Ser. No. 07/053,950, filed 05/26/87, for example, disclose nuclear-based explosive detection systems that make use of neutrons of mainly thermal energies. In contrast, European Patent publication EP-O-227-497-A1 discloses a nuclear-based explosive detection system wherein fast neutrons of energies from 7 to 14 million electron volts (MeV) are employed. Disadvantageously, the thermal neutron based detection systems provide, for practical purposes, primarily only one signature of the four cardinal constituents of explosives (i.e., the elements hydrogen, carbon, nitrogen, and oxygen), namely the signature of nitrogen (and possibly hydrogen). The fast neutron based detection system, on the other hand, may provide signatures of all four ingredients of explosives, or other contraband, thus enhancing the interrogating power of the fast neutron contraband detection systems.
It must be observed, however, that simply obtaining the signatures of the constituent elements of a specified contraband does not necessarily indicate that such contraband is present in the object under investigation. This is because many benign materials (non-contraband) also include such elements. A great diagnostic advantage may thus be obtained when a three-dimensional image of the distribution of element densities within the interrogated body is also formed, as such image may help further distinguish contraband from non-contraband. A suitable three-dimensional image for this purpose may advantageously be obtained by performing a section-by-section neutron irradiation of the object, and by performing a computer-based analysis of the energy and intensity of the signals that are produced from each section. Such analysis requires the judicious positioning of gamma-ray detectors around the object, as taught in Applicants' earlier patent application, Ser. No. 07/053,950, filed 05/26/87, which application is incorporated herein by reference.
A viable contraband detection system should meet several requirements. These requirements include: (1) the detection of explosives or other contraband should be independent of the specific configuration of the explosive or contraband (i.e., the explosive or other contraband must be detected regardless of its shape); (2) the examination of the object must be non-intrusive in order to assure maximum privacy of the contents of the object under investigation and maximum throughput of objects through the system; (3) the detection system must provide a high probability of detection, i.e., a high detection sensitivity, and a low rate of false alarms; (4)the detection technique must be non-hazardous to the objects being interrogated, the operating personnel, and the environment; and (5) the detection system must be reliable, easily operated and maintained, and capable of functioning in a variety of environments.
It is noted that non-nuclear explosive detection systems are also known in the art, some of which are mentioned in the above-cited patent application. However, to date, these non-nuclear systems by themselves have not been able to comply with the above requirements.
Nuclear-based explosive detection systems, on the other hand, are able to address most of the needs of a viable detection system as set forth above, but existing nuclear-based systems still fall short in some areas. The present invention advantageously addresses specific improvements in the nuclear-based detection field that overcome the shortcomings of the prior art systems. Such improvements can be better appreciated and understood by first reviewing the relevant properties of explosives, and then assessing the shortcomings of the prior art detection systems in detecting such explosives. (It is to be emphasized, of course, that explosives are just one example of a particular type of contraband that could be detected using the present invention.)
Explosives may generally be divided into 6 types:
1) Nitroglycerine based dynamite, PA0 2) Ammonium nitrate based dynamite, PA0 3) Military explosives (Composition-4, TNT, PETN, and picric acid), PA0 4) Homemade explosives (made, e.g., of fertilizer, fuel oil), PA0 5) Low order powders (e.g., black, and smokeless powder), and PA0 6) Special purpose explosives (e.g., lead azide, lead styphanate, mercury fulminate, and blasting gel).
The physical properties and the elemental compositions of these explosives are summarized Table 1. One finds that the nominal density of explosives is typically 1.6 g/cm.sup.3 and ranges from 1.25 g/cm.sup.3 to 2.0 g/cm.sup.3 and more, and the predominant elemental components are hydrogen, carbon, nitrogen, and oxygen. Reading Table 1, one should keep in mind that an explosive must have a minimum propagation thickness in order to be effective, thus requiring minimum sizes of contiguous explosive bodies.
U.S. Pat. No. 4,756,866 (Alvarez) teaches an explosive detection system that uses an inert tracer, e.g., deuterium, implanted in explosives at the time of their manufacture. The illicit traffic in explosives is then detected by irradiating the luggage and parcels with photons of energy greater than 2.223 MeV and detecting neutrons resulting from the photo-disintegration of the implanted deuterons. The main drawbacks of this approach are (1) a global consent among manufacturers of explosives would be required to add adequate amounts of deuterium to the explosives, and (2) some explosives, e.g., black powder (which contains no hydrogen) and the homemade explosives, would escape detection.
Another nuclear technique suggested in the art for detecting explosives involves recognizing that nitrogen is the major component in explosives, see Table 1, and then using the production of radioactive .sup.13 N (t.sub.1/2 =10 m, positron emitter) in the .sup.14 N(.gamma.,n) process, induced by photons of energy greater than 10.6 MeV, and the subsequent detection of annihilation radiation (facilitating the positron emission tomography), to identify the presence of nitrogen. However, the prohibitive factor associated with this technique is the large radiation doses (on the order of krad/kg) that are inevitably delivered to the irradiated objects, which radiation doses create an unacceptable hazard to the public.
A related nuclear technique that overcomes, or at least minimizes, the aforementioned radiation problem is based on activation of nitrogen with thermal neutrons, as taught, e.g., in the aforementioned U.S. Pat. No. 3,832,545 (Bartko) and the above-cited U.S. patent application Ser. No. 07/053,950 (the '950 application). Both inventions draw heavily on the fact that the .sup.14 N(n,.gamma.) process, initiated with slow neutrons, may give rise to prompt gamma-ray photon emission of precisely 10.8 MeV, thus greatly facilitating its detection. While sharing this basic premise, however, the embodiments of these two inventions differ substantially. For instance, in the Bartko patent, organic scintillators are used as gamma-ray detectors, with the result that a rather moderate source position resolution (i.e., a poor image of the nitrogen distribution), and a low detection efficiency are obtained.
The '950 application, on the other hand, teaches the use of judiciously positioned arrays of inorganic scintillators, e.g., NaI(T1), viewing the moving objects that are immersed in the bath of thermal neutrons. A computer-based analysis of the measured spectra from the individual detectors provides a quite good three-dimensional image of the nitrogen density in the object, thus facilitating, in principle, the detection of concealed explosives. Explosive detection systems developed from the invention described in the '950 application have, in fact, satisfied the Federal Aviation Agency (FAA) requirements for explosive detection in 1989, and are currently being tested at selected airports.
However, even the invention described in the '950 application is not free from two systemic deficiencies. First, the thermal neutron bath is not homogeneous, since the thermal neutrons tend to be depleted by the object material. Accordingly, the nitrogen detection efficiency is diminished in the inner part of the object volume. Secondly, an inference of presence of the explosive in an object merely due to an elevated density of nitrogen alone is bound to cause frequent false alarms. (Refer, for example, to the nitrogen content in wool, leather, food stuff and other benign commodities included in Table 1.)
Fortunately, however, the study of the performance of the embodiment of invention described in the '950 application provided the impetus for the present invention, which invention advantageously ameliorates the cited shortcomings inherent to the Bartko invention and the deficiencies of the invention described in the '950 application.
A still further prior art nuclear-based technique for detecting explosives is referenced in European patent publication EP O-227-497-A1. This document describes an explosive detection system based on inelastic scattering of 7-14 MeV (fast) neutrons. The fast neutrons are produced in the .sup.3 H(d,n).sup.4 He reaction with a pulsed deuteron beam, and the prompt gamma-rays due to the neutron interactions are detected with a solid state diode [HPG], outputs of which are appropriately timed. An analysis of the prompt gamma-ray spectra provides an indication of the concentrations of elements in the irradiated objects. In particular, it is stated that the measurement of the ratio of the intensities of the prompt gamma-ray transitions in .sup.14 N to the intensities of transition(s) in .sup.16 O yields information regarding the presence of explosive materials.
Although the specifics of the fast neutron invention described in the European patent document are very scant, from the description given it appears that the nitrogen to oxygen ratio is used as the sole indicator of the presence of an explosive material, and the carbon signal is ignored. This appears to be due to the fact that the carbon signal is represented by a very broad line in the radiation spectrum, caused by the considerable recoil velocity of carbon nuclei and the short lifetime of the 4.44 MeV level in .sup.12 C, and this broad line is difficult to measure using high resolution solid state detectors of the type proposed.
Another deficiency associated with the fast neutron device cited in the European document appears to be the necessity of using rather long irradiation times of the objects under examination, resulting in a relatively slow throughput time of the explosive detection system. This long irradiation time is due in large part to the inherently low detection efficiency of the high resolution solid state gamma-ray detectors, created by two technical limits of these detectors: (1) the relatively small active volumes (e.g.,less than 100 cm.sup.3) of the detectors, and (2) the upper bandwidth of the detectors, resulting in maximum count rates less than 10.sup.5 Hz. What is needed therefore is a fast neutron system that is not encumbered by these detection limitations.
Even if a fast neutron system is obtained that overcomes these identified deficiencies, however, there are still other adverse affects associated with the use of fast neutrons that must be addressed before a viable contraband detection system based on fast neutron activation may be realized. For example, the energy resolution of gamma-ray detectors of intrinsic germanium deteriorates after a fluence of .apprxeq.10.sup.10 n/cm.sup.2 of fast neutrons, and a detector annealing cure must be applied or detectors replaced. The invention disclosed in the European patent document teaches, for example, the use of a shadow shield of tungsten and borated polyethylene in order to reduce this neutron dose. However, assuming the length of the indicated shadow bar to be 0.5 m, it is estimated that this amount of shielding will result in an attenuation factor greater than 2.6.times.10.sup.-3. At this level of attenuation, and assuming that a d+T neutron source of yield of 10.sup.12 n/second is employed, it can be shown that the availability of the detector unit will be limited to less than 47 hours. Unfortunately, this is an unacceptably short time for a viable contraband detection system.
TABLE 1 __________________________________________________________________________ Various Physical Properties and Approximate Composition of Explosives and Other Materials PHYSICAL DENSITY % Weight Composition MATERIAL STATE (G/CM.sup.3) H C N O OTHER O + N __________________________________________________________________________ Nitroglycerine Liquid 1.6 2.2 15.9 18.5 63.4 0 81.9 (NG) EGDN Liquid 1.48 2.4 22.0 17.1 58.5 0 75.6 Amn. Nitrate Solid 1.7 5.0 0 35.0 58.0 0 93 Black Powder Solid 1.7-1.95 0 .about.22 10 36 S(3),K(29) .about.46 Nitrocellulose Solid 1.50-1.7 2.4 24.3 14.1 59.2 0 73.3 (9-14% N) PEIN (Pure) Solid 1.76 2.4 19.0 17.7 60.7 0 78.4 PEIN (Data Sheet) Solid 1.48 4.3 31.4 12.2 52.1 0 64.3 TNT (Pressed) Solid 1.63 2.2 37.0 18.5 42.3 0 60.5 Composition B Solid 1.71 2.7 24.4 30.5 42.7 0 73.2 Lead Styphnate Solid 3.02 0.7 15.4 9.0 30.8 Pb: 44.2 39.8 Tetryl Solid 1.57-1.71 1.8 29.3 24.4 44.6 0 69 Dynamite Solid 1.25 4.0 14.0 15.-20 59.0 Na: 10.0 74-79 Octogen (HMX) Solid 1.90 2.8 16.2 37.8 43.2 0 81 Composition 3 Putty-like 1.58-1.62 2.9 22.8 32.8 41.6 0 74.4 (C-3) Solid Composition 4 Putty-like 1.64-1.66 3.6 21.9 34.5 40.2 0 74.7 (C-4) Solid Picric Acid Solid 1.76 1.3 31.4 18.3 48.9 0 67.3 Lead Azide Solid 4.48 0 0 28.9 0 Pb 28.4 (Detonator) Triacetone Solid 1(?) 9.7 38.7 0 51.6 0 59.7 Triperoxide Hexametylene Solid 1.57 5.77 34.6 13.5 46.2 0 59.7 Triperoxide Diamine NON EXPLOSIVE Packed Clothes Solid &lt;0.1 Polyester Solid (1.38) 3.7 66.7 0 29.6 0 29.6 Dacron Solid (1.38) 4.2 62.5 0 33.3 0 33.3 Cotton Solid (1.30) 6.0 48.0 0 46.0 0 46.0 Wool Solid (1.32) 4.7 37.5 21.9 5.1 0 27.0 Silk Solid (1.25) 5.3 39.5 28.8 26.3 0 55.1 Nylon Solid (1.14) 9.7 63.7 12.4 14.2 0 26.6 Orlon, Acrylan Solid (1.16) 5.7 67.9 26.4 0 0 26.4 Other Materials ABS (Acetonitrile Solid 1.20 8.92 84.5 76.5 0 0 76.5 Butadiene Styrene) Melamine- Solid 1.48 5.5 43.6 50.9 0 0 50.9 Formaldehyde Neoprene (Wet Solid 1.25 4.4 64.0 0 0 Cl: 31.6 0 Suites) Polyurethane Solid 1.50 7.9 52.2 12.2 27.8 0 40 Polyethylene Solid 0.92-0.96 14.3 85.7 0 0 0 0 Polypropylene Solid 0.89-0.91 14.3 85.7 0 0 0 0 Lucite, Acrylic Solid 1.16 9.1 54.6 0 36.4 0 36.4 Plexiglass PVC Solid 1.2-1.55 4.8 38.4 0 0 Cl: 56.8 0 Saran Solid 1-1.7 3.1 30.0 0 0 Cl: 66.9 0 Water Liquid 1 11.1 0 0 89.9 0 89.9 Ethyl Alcohol Liquid 0.79 13.1 52.1 0 34.0 0 34.0 Sugar Solid 1.59 6.5 42.0 0 51.4 0 51.4 __________________________________________________________________________